Select the Trigger Filter "Fills Out Form." Select the name of the form that you want to use from the drop down.

Next, we will add a filter to the smart list. For example, say you only want people who filled out the form with a State field value of NC to be added to the list. You will need to add the state filter field to your smart list and select NC. (See screenshot)

Next, you will need to set up your flow step.

Under your flow step select the Add to List option and drag it into the workspace. Select the list that you want to add these people to from the drop down.

Lastly, under the Schedule tab select the Activate button

Once the smart campaign is activated you should be good to go. Just make sure you test the smart campaign out to ensure it's working correctly.